Cycle length calculator
Add the start dates of your last few periods. You get your average, the spread, and whether it is regular by the usual definition.
Your details
Your average cycle
28.7
days, from 3 cycles
- Cycles29, 28, 29 days
- Shortest to longest1 day
- Next period, by your average19 Nov
- The two after that18 Dec and 16 Jan
- Likely fertile days this cycle (calendar method)cycle days 10 to 18, 30 Oct to 7 Nov; a rough guide for trying to conceive, not contraception
- Add it to your calendarDownload the next 3 period dates (.ics)
Within the usual range for frequency and regularity. Cycles vary with stress, illness, weight changes and age, especially in the teens and the 40s. The calendar method needs at least 6 cycles to be used to avoid pregnancy, and even then it is one of the less reliable methods.
Periods that are very heavy, very painful, come less than 21 or more than 40 days apart, or bleeding between periods or after sex: see your GP.
The working
The gaps between starts
Count the days from one start to the next. Four dates give three cycles; clear a box if you have fewer. Gaps of 29, 28 and 29 average 28.7 days, with a spread of 1 day: very regular.
Normal limits
FIGO 2018
| Measure | Usual |
|---|---|
| How often | every 24 to 38 days |
| Shortest to longest | no more than 7 to 9 days apart |
| Length of bleeding | up to 8 days |
Tracking
Getting a good picture
- Note day 1 every time, even if light
- Spotting before a period is not day 1
- Three to six cycles give a fair average
- Bring the dates to any GP appointment
Looking ahead
Next periods and the calendar method
The page projects your next three periods from your average and lets you download them as calendar dates. It also shows the calendar method fertile window from the WHO Family Planning Handbook: the shortest cycle minus 18 gives the first likely fertile day, and the longest minus 11 gives the last. With cycles of 28 and 29 days that is cycle days 10 to 18.
This is a rough guide for people trying to conceive. As contraception the calendar method needs at least 6 recorded cycles and is one of the less reliable methods; NHS contraception services can talk you through better options.
Read with care
What this cannot tell you
- CausesIt cannot say why cycles are irregular.
- Teens and 40sCycles are often irregular at both ends of the reproductive years.
- PregnancyA very late period could mean pregnancy.
Questions
Common questions
How do I calculate my cycle length?
Count from the first day of one period to the day before the next starts.
What counts as an irregular cycle?
A gap of more than 7 to 9 days between your shortest and longest cycles.
Is a 25 day cycle normal?
Yes. Anything from 24 to 38 days is within the usual range.
Sources
- WHO and Johns Hopkins CCP. Family Planning: A Global Handbook for Providers, calendar rhythm method
- Munro MG et al. FIGO systems for normal and abnormal uterine bleeding, 2018 revision. Int J Gynaecol Obstet
- NHS: periods and the menstrual cycle
Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.
Support when you need it
Contact your midwife or GP any time you are worried, and call NHS 111 if you cannot reach them. If a date or a result brings up difficult feelings, Tommy's and the Miscarriage Association offer kind, free support.
Related
Related calculators
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- Ovulation calculatorYour likely ovulation day and fertile days.Open
- Perimenopause cycle trackerHow your cycle lengths are changing.Open
- Fertile window calculatorYour next three fertile windows.Open
- PCOS lifestyle goal plannerSmall activity goals for PCOS.Open
- PMS preparation plannerYour likely PMS days, three months ahead.Open